var i=0,
    aBtn=document.querySelectorAll(".playBtn li"),
    aContent=document.querySelectorAll(".playPic li");
for(;i<aBtn.length;i++){
    var index=0;
    aBtn[i].index=i;
    aBtn[i].onmouseenter=function () {
        aBtn[index].className="";
        aContent[index].className="";
        index=this.index;
        aBtn[index].className="active";
        aContent[index].className="show";
    }
}